Large items such as airplanes are constructed from master models. In modern times, parts for the items are designed on computer and the master models are made from the computerized data sets describing the surfaces of the items.
Before computers became commonly used, however, these master models were often plaster and plywood models made by hand. For instance, the Boeing Company, the assignee of the present invention, has many large plaster master models for airplane wings and other large parts stored in various warehouses across the country, thereby incurring expensive storage and maintenance costs. Since most or all of these were handmade master models, there are no data sets for the parts, and the parts must be reverse engineered in order to computerize the master models and thereby ensure against the consequences of the loss or damage to the master models. A portable, high-accuracy, digitizing system is needed so that surface maps of these handmade master models could be generated in situ, thereby avoiding the risk and expense of transporting large, irreplaceable parts to a scanning site. After a data set has been created and archived, the master model is no longer needed and may be destroyed, thereby eliminating the warehousing costs.
In addition to reverse engineering parts, digitizing systems are also used to inspect manufactured parts to ensure that they have been made within the required tolerances, for product design, and for tooling development.
Many devices exist for creating surface scans, and may be either contact or non-contact systems. Contact systems use a touch probe for directly measuring a part, or a laser measuring distance to a target placed in contact with the surface of a part to be measured. While contact systems are very accurate, resolution is limited by the size of the probe (typically a 3 millimeter (1/8th inch) ball) or target (e.g. a 38 millimeter (1.5 inch) retroreflector). Additionally, use of contact systems to measure parts having soft or non-rigid surfaces creates a danger of altering or deforming the surface being measured. Examples of contact systems include Coordinate Measurement Machines (CMM), Computer Aided Theodolites (CAT), and laser tracking systems.
Non-contact systems typically use lasers such as in laser digitizing heads, or other optical devices, and do not come into contact with the surface of the part being measured.
While many dimensional inspection devices exist, very few can accurately handle large parts or parts with complex geometric shapes with high resolution. Those that will handle large parts require expensive temperature and humidity controlled environments for maximum accuracy.
Coordinate Measurement Machines (CMM) such as the Chameleon and Xcel models manufactured by Brown & Sharpe of North Kingstown, R.I., the Delta, Bravo and Lambda models manufactured by DEA-Brown & Sharpe SpA of Torino, Italy, and the Bright series manufactured by Mitutoyo U.S. of Aurora, Ill. are common throughout the aerospace industry for dimensional inspection. They generally consist of a large granite block base and precision three axis motion actuators. To assure their accuracy, they are usually found in controlled environment chambers, and installed on floors that have been isolated from vibrations. CMM's use touch probes of known lengths and diameters that contact the part, and the X, Y, and Z axis positions are recorded for future reviewing. In addition to not being portable, problems with CMMs include high facilities and maintenance costs, mechanical limitations, and the fact that their accuracy is not assured with large parts because a slight temperature or humidity change is cumulative over the CMM work envelope.
Three and five axis mills are also used for mapping the surface of a part. The same style of touch probe used on the CMM's can be used on the mills to take measurements. When using a mill to measure a part, the touch probe is moved over the surface of the part by a human operator manipulating the controls of the mill Z offset data is collected by the touch probe, and the X, Y, and Z axis positions are obtained from the mill's internal mechanical position sensors (e.g. a certain number of motor rotations will equate to a known distance of movement). The problems with this system include the lack of portability and thermal inaccuracies of the CMM and, additionally, if the same machine that was used for machining the part is later used for measurement, inaccuracies in the mill may be hidden from the measurement if the machine and part to be measured are in the same relative positions. If the machine and part to be measured are not in the same relative positions, or if a different machine is used for measuring than was used during machining, inaccuracies in the mill can overemphasize errors, or report errors where none exist. In any case, there is no assurance of accurately measuring and recording part dimensional attributes.
Photogrammetry systems such as the models SD2000 and SD3000 manufactured by Leica AG of Heerbrugg, Switzerland, and the FotoG-FMS manufactured by Vexcel of Boulder, Colo., theodolites such as the models NA820, NA824, and NA828 manufactured by Leica AG of Heerbrugg, Switzerland and the model NT-4D manufactured by Nikon, USA of Melville, N.Y. and computer aided theodolites (CAT) such as the model T460 manufactured by Leica AG of Heerbrugg, Switzerland, the models GTS-500 and GTS-700 manufactured by Topcon of Tokyo, Japan, and the model DTM-400 manufactured by Nikon USA of Melville, N.Y. all utilize optical devices with a known relationship to triangulate the X, Y, and Z positions of targets placed on the part being inspected. All three systems are portable systems capable of measuring large and complex parts accurately, but they are time and labor consuming because of the necessity of laying out a large number of targets on the part, and the time to convert the data into a usable format. For instance, for a part having a thirty square foot surface, three days are generally required to lay out and measure a part using a six inch grid and to return the X, Y, Z data. Larger parts and/or smaller grids will take considerably more time.
The Hyscan laser digitizing head manufactured by Hymarc Ltd., of Ottawa, Ontario, Canada, is a non-contact system which is designed to be mounted onto a CMM, or some other translation device such as a mill. Measurement is performed by replacing the touch probe with the Hyscan digitizing head. Instead of using a touch probe to mechanically measure the distance to a point on the surface of the part being measured, the Hyscan uses a rastering laser beam and local triangulation to calculate offset values to the surface of the part being measured. The Hyscan laser scanner captures surface data at a continuous rate of 6,144 points per second with 0.025 millimeter (0.001 inch) accuracy. The data is acquired point-by-point using a swinging laser beam which scans back and forth in a pendulum-like motion and rapidly "paints" out the surface features in high resolution. This is done using a system of a synchronized moving mirror and precise triangulation techniques to calculate each point. The moving mirror sweeps the laser beam across the part. The beam is reflected from the surface of the part through a series of mirrors and a lens onto a Charged Coupled Device (CCD) array. The location on the CCD array that the reflected laser beam hits is translated into offset values which are combined with the host's internal X, Y, and Z coordinates to create a data set. While the Hyscan digitizing head collects high accuracy offset data in high resolution, it is dependent upon the CMM, or other translation device for overall accuracy and measurement envelope.
Robotic Vision Systems, Inc. (RVSI) of Hauppauge, N.Y. manufactures a three axis gantry with a non-contact split beam laser digitizing head. The head provides a Z-offset value that is added to the gantry's internal X, Y, and Z positions. Like the CMM's, the facility and maintenance costs are high because of the thermal inaccuracies, and like the Hyscan, the RVSI system is dependent upon the gantry's internal, mechanical, position sensors for accuracy.
In addition to dedicated scanning systems, 3-D laser tracking systems such as the Smart 310, manufactured by Leica AG of Heerbrugg, Switzerland, have been employed to scan parts. Laser tracking systems are high accuracy, but low resolution, contact systems that use a portable post mounted laser to follow and return position data of a retroreflector placed on the surface being scanned. Surfaces are measured by manually moving the retroreflector across the surface to be measured, thereby creating the possibility of human positioning errors reducing accuracy. Movement of the retroreflector is tracked by projecting a laser beam from the laser head to the retroreflector. The laser beam is reflected back to the laser head almost directly along its transmission path as long as the retroreflector does not move. When the retroreflector is moved, the laser beam no longer hits the optical center of the retroreflector. Instead, the retroreflector causes the reflected beam to follow a parallel path back to the laser head. The parallel offset between the transmitted and return laser beams is used to determine the distance and direction that the retroreflector has been moved. The parallel offset is determined at the position detector, which is a two-dimensional position sensitive photo diode within the measuring head. The parallel offset information is also used by the laser tracking system in order to point the laser beam back on the center of the retroreflector. The Smart 310 is capable of performing 1,000 such updates per second, thereby allowing continuous tracking of any path of retroreflector movement.
Surface scan measurements are made using an interferometer in the laser tracking system. As with all interferometers, no absolute distances can be determined. It is only possible to determine changes in distance because measurements are made by means of fringe counting. Therefore, in order to use a laser tracking system with an interferometer to measure absolute distances, measurements must always begin with the retroreflector positioned at a point to which the absolute distance is known. The interferometer fringe counting pulses are then added to or subtracted from this initial distance to obtain actual distance measurements. Since the laser tracking system requires the retroreflector for accuracy, the size of the retroreflector, which has a typical diameter of 38 millimeters (1.5 inches) or larger, limits the size of the surface detail which can be detected, i.e. changes in surface detail which are smaller than the retroreflector cannot be detected because the retroreflector cannot be fit in the surface detail.
None of the presently available dimensional inspection devices offers a combination of low cost, portability, high resolution, high-accuracy, and the ability to handle large parts or parts with complex geometric shapes. Thus, there exists a need for a dimensional inspection device that is portable and not time or labor consuming to set up, and that can handle large parts and/or parts with complex geometric shapes with high-accuracy and high resolution.